BUG-5280: introduce Request/Response and related concepts

BUG-5280: introduce Request/Response and related concepts

This adds a versioned message concept and Request/Response concepts
built on top of it. Serialization uses the serializable proxy pattern,
with one proxy class per ABI version.

For ActorRefs we rely on the fact that serialization of these
messages should only ever occur in the context of JavaSerializer,
so we can use its #currentSystem() and perform ActorRef serialization
without having to go through writeObject().

Change-Id: Ic2701646bf3fc34c0d8d1dea278bbf63324a6786
Signed-off-by: Robert Varga <rovarga@cisco.com>